The present invention relates to a fail-safe apparatus which is useful with an anti-skid braking system to prevent a locked wheel condition and ensure safety in stopping a vehicle.
Generally, well known prior art anti-skid systems for air braking systems comprise: a sensor for detecting the revolution of an appropriate member (for example, axles, propeller shafts, etc.) which rotates with rotation of the axles; control circuitry for determining whether or not the thus sensed revolution is suitable and then producing braking pressure control signals; and a control valve associated with the air braking system for controlling braking pressure in response to the signals from the control circuitry.
Within these prior art anti-skid systems, there are provided fail-safe means which sound the alarm or render the anti-skid systems inoperative to avoid a serious danger when the system takes its abnormal status regardless of the cause. It is, of course, highly desirable that the fail-safe means be responsive to any abnormal status in the sensor, the control circuitry and the control valve. Specifically, in order to determine whether the valve is normal or abnormal with respect to its operation, it is required to detect even a small amount of a stroke of the valve. However, many difficulties are experienced in obtaining such detection.